Solubilization of Micellar Cubic Phases and Their Structural Relationships in Anionic−Cationic Surfactant−Dodecane−Water Systems

Abstract

The morphology and solubilization of micellar cubic phases are investigated as a function of the cationic weight fraction in the mixture of two surfactants, the surfactant-to-water ratio, and the oil weight fraction in the water−dodecane−sodium dodecyl sulfate (SDS)−dodecyltrimethylammonium bromide (DTAB) or didodecyl-dimethylammonium bromide (DDAB) systems at 25 °C. One phase transition H1 → I1 appears at low cationic weight fraction, Y, and the other Lα → I1 takes place at high Y upon addition of oil. Solubilization for dodecane of the micellar solution and of the micellar cubic phase increases with an increase in the cationic surfactant weight fraction Y from 0 to 0.30. SAXS measurements index the structure of the cubic phase as the discontinuous Pm3n cubic phase for single surfactant systems and mixed surfactant systems at different levels of oil. The cross-sectional area of the hydrophilic head of the surfactant keeps constant with increase in the oil weight fraction at a fixed cationic surfactant weight fraction as the interlayer spacing of the Pm3n cubic phase greatly increases. However, it decreases with an increase in the cationic surfactant weight fraction at fixed oil weight fraction as the interlayer spacing of the Pm3n cubic phase slowly increases. So the solubilization for oil can be increased by adjusting the mean curvature of the surfactant interface layer with the addition of mixed surfactants and long-chain oil.